Tank blanketing is an effective way to prevent fires in storage tanks which contain flammable liquids. Tank blanketing prevents the formation of an explosive vapor/air mixture. A blanket or pad of inert gas in a vapor space inside a tank prevents atmospheric air from entering the tank. In addition to preventing outside air, which contains oxygen, moisture and other contaminants from entering the tank, the blanket also minimizes evaporation of the stored product to reduce product emission levels.
The inert gas, usually Nitrogen, is injected as necessary, into the vapor space in the tank to maintain an inert atmosphere. The blanket pressure is usually very low (less than 1 PSI).
The inert gas is admitted only during inbreathing cycles (vacuum cycles). An inbreathing cycle occurs when liquid is being withdrawn from a tank, or when vapors condense in a tank due to a decrease in temperature. The amount of inert gas required for a specific application is based on the maximum inbreathing demand under conditions of sudden cooling, caused by weather conditions, and the emptying rate of the tank.
Although Nitrogen is most commonly used, other gases, including natural gas, may be used in some applications. Depending upon the specific application, the selection of an inert gas for a particular process is based upon at least one of the following characteristics:
1. Non-flammable in the desired atmosphere PA1 2. Non-interacting with the tank liquid PA1 3. Chemically inactive PA1 4. Non-toxic PA1 5. Availability in large quantities PA1 6. Economic factors
In designing a tank blanketing system, several considerations are evaluated:
1. The tanks themselves as well as the piping, dikes, fittings, etc., should be designed and constructed in accordance with sound engineering principles.
2. Inert gas or tank blanketing equipment must be completely reliable and capable of maintaining an adequate supply of inert gas at all times. Equipment breakdown or equipment that is inadequately sized for the job will result in a higher oxygen concentration in the vapor space of the storage tank which would mix with the vapors resulting in a potentially flammable mixture.
3. The inert gas should be introduced into the tank so that it is distributed effectively.
4. Contamination of the inert gas from any source must be guarded against. Suitable devices should be employed to hold moisture to an absolute minimum.
A blanketing valve is used in tank blanketing applications to regulate the pressure of the inert gas layer on top of a tank liquid. The valve senses the pressure of the tank blanket and opens to allow flow in of more inert gas when pressure drops below a set pressure. The valve closes and stops flow when pressure builds back up to the set pressure in the tank.